The invention relates to underwater telecommunications systems, underwater power networks for supplying power to underwater equipment, to underwater repeaters, to methods of installing such apparatus, and to methods of upgrading such apparatus.
It is known to provide telecommunications systems having equipment underwater for various purposes.
One application is for data transmission between land sites separated by water. Other applications include telemetry or monitoring of underwater installations for other purposes. Conventional underwater data transmission systems started with metallic conductors arranged in a coaxial cable. Fibre optic transmission systems were later introduced. For spans beyond several hundred kilometers, repeaters were required. These started with optical to electrical to optical regenerators. In the 1990""s, optical amplifier systems were introduced, to avoid the conversion into the electrical domain.
In corresponding terrestrial optical data transmission systems, the capacity has been steadily increasing, as data rates have increased up to 10 G bits per second, and the number of channels wavelength multiplexed together on the same fibre, has increased into the hundreds. A typical terrestrial fibre route is laid with 48 pairs of fibres, many of the fibres being unused initially, but laid to allow for future expansion.
Underwater systems have not matched the growth in capacity of such terrestrial systems. The two primary reasons are the limitation in the amount of power which can be fed to underwater repeaters, such as optical amplifiers or regenerators, and the extremely high reliability requirements for such underwater equipment. The reliability is required because the underwater equipment is so inaccessible for maintenance or repair, once installed. In particular, optical pumps for optical amplifiers may be responsible for much of the power consumption, e.g. in the order of 50%. Notably, these components may be amongst the least reliable, depending on the pump power level and other factors. For a long haul system requiring many optical amplifiers, every 50 to 100 kilometers, the power for each of these optical amplifiers is supplied along the same cable as contains the fibres for carrying the data. Typically, each repeater housing, containing one optical amplifier for each fibre, uses 0.5 to 1.5 amps, and drops in the region of 30 to 50 volts. Each of the amplifiers are connected in series along the cable.
Thus for a transoceanic route of thousands of Kilometers, the total voltage drop will run into thousands of volts for the optical amplifiers. The voltage drop caused by losses in the copper power conductor between optical amplifiers maybe in the same order of magnitude.
If the losses in the copper are reduced by using thicker copper, the weight of the cable increases prohibitively. If more repeaters are added, the total voltage drop increases, and the insulation of the cable and the repeaters, to withstand such high voltages, becomes prohibitively heavy.
For these reasons, often only 8 to 12 fibre pairs are installed in the cable, because it is impossible to supply more power for sufficient optical amplifiers for more fibres. Efforts to improve the capacity have involved trying to reduce the power consumption of each optical amplifier, by using more efficient optical pumps, and using more power efficient semiconductors for the control electronics.
An example of a power supply network for an underwater transmission system is shown at pages 44-46 IEEE Communications Magazine February 1996.
An object of the invention is to address at least some of the above limitations. According to a first aspect of the invention, there is provided an underwater telecommunications system having a first underwater cable for carrying data traffic, one or more underwater repeaters coupled to the first cable, and an underwater power network coupled to the repeaters for supplying power from a remote power source, to the repeaters, at least part of the power network extending along a second underwater cable, separate from the first cable.
Several advantages can arise from providing a separate cable for some or all of the power supply. Firstly it enables the routing of the cable for data traffic and of the second cable for power supply, to be independently optimised. In particular the power route may be made shorter or multiple power feeds used. This enables more power to be delivered to the repeaters, which enables more repeaters, more functionality in the repeaters, and more fibre pairs. More functionality in each repeater can enable more capacity (through more channels and/or higher data rates per channel).
Thus if for example the nearest land to the midpoint of the data route is not at the data terminals, then the route for the second cable carrying power, may be shorter than the data route. If there is a string of repeaters spread along the data route, it becomes possible to route power directly to repeaters in the middle of the string rather than being restricted to supplying it through all the repeaters in series. Thus a critical limitation of the prior art can be overcome, with very significant implications for overall system performance, capacity and costs.
Alternatively, or as well, the electrical voltage and or current can be reduced, and thus cause the weight of insulation material and or the weight of copper in any cable used for power supply, to be reduced, thus reducing cost and easing installation.
A second advantage is that separate cables can be maintained or upgraded independently. Thirdly, different levels of redundancy can be provided for the separate cables. Fourthly, different levels of mechanical strength can be designed, as appropriate for each cable.
An preferred example has the cable for carrying the data having at least one optical fibre for carrying the data.
Another preferred example has a transmitting data terminal on land or surface at one end of the first cable, and a receiving data terminal on land or surface, at the other end.
In another preferred example, power is supplied along both cables. This can enable the second cable to supply power to many repeaters along the first cable, or provide redundancy of power supply.
Another preferred example has a string of repeaters at intervals along the first cable, the power network being coupled to supply power at one or more intermediate locations along the string, to divide the string into two or more separately powered substrings. This has the advantage that each of the substrings may be shortened and thus more power can be delivered to each substring, and the data carrying capacity increased. Alternatively, or as well, the length of the cable can be increased.
In another preferred example, power supply in one substring can be supplemented or replaced by supplies to neighbouring substrings. This enables provision for redundancy to be built in, to improve reliability against failures such as cable cuts. Also, it enables provision for upgrading to deliver more power to selected substrings.
Another preferred example has two or more data carrying cables, the second cable being coupled to supply power to repeaters in the data carrying cables. This enables more significant cost advantages to be achieved by reducing the number of power cables required.
Another preferred example has a coupling arrangement for enabling power to be coupled from the second cable to the repeater or repeaters after either cable has been laid underwater. This may ease installation, and allow later upgrade or expansion to be carried out.
In another preferred example the coupling arrangement has one or more tails branched off the first or the second cable. This can ease installation, or ease later upgrade or expansion, because it is easier to splice to a tail than splice into a cable.
Another preferred example has a tail for coupling the power from the second cable to the repeaters of the first cable, the second cable and the first cable each having a branch to couple the tail. This is a preferred alternative to a four way junction box because it keeps the cables more independent, which may ease installation and upgrading, and because branching technology is tried and tested.
Another preferred example is arranged such that at least some of the repeaters are coupled to the second cable to receive all their power from the second cable such that no power need be transmitted along at least some parts of the first cable. This can enable part or all of the first cable to be constructed of lighter cable.
In another preferred example, the second cable has a series of junction boxes at intervals, arranged to allow more repeaters to be coupled after installation of the second cable.
Another preferred example has the power network being coupled to the first cable at two or more locations and arranged to provide redundancy such that a failure of supply can be compensated by adjusting the power supply at any of the two or more locations.
A second aspect of the invention provides an underwater power network for supplying power to underwater repeaters of the above mentioned telecommunication system, the power network comprising a power terminal on land or surface, and the second cable extending underwater form the power terminal, separate from the first cable for carrying data, and having coupling assemblies for coupling power to the repeaters.
A preferred example of the power network has the second cable being arranged to supply power to repeaters on two or more cables carrying data traffic.
Another preferred example has further second cables for supplying power to different locations along the first cable.
A third aspect of the invention provides an underwater repeater for transmitting data traffic along cables in a telecommunications system, the repeater having three or more cable couplings, two of the cable couplings arranged to carry the data traffic, a third of the cable couplings arranged to couple power to the repeater, not the data traffic.
A fourth aspect of the invention provides an underwater telecommunications system having a first underwater cable for carrying data traffic, underwater telecommunications equipment coupled to the first cable, and an underwater power network coupled to the underwater equipment, for supplying power to the equipment from land or surface, along a second underwater cable, separate from the first cable. Corresponding advantages apply to equipment other than repeaters.
A fifth aspect of the invention provides a method of using the above mentioned system to transmit data traffic, comprising the step of passing the data traffic to a terminal of the system for transmission over the system.
A sixth aspect of the invention provides a method of installing the above mentioned system, comprising the step of laying the first and second cables separately, and the step of coupling the second cable to enable it to supply power to the repeaters.
A seventh aspect of the invention provides a method of upgrading the system, comprising the steps of laying a further first cable, and coupling the further cable to the second cable to enable power supply from the second cable to the repeaters of the further first cable.
An eighth aspect of the invention provides an underwater telecommunications system having a first underwater cable for carrying data traffic, one or more underwater repeaters coupled to the first cable, and one or more power couplers for coupling a second underwater cable for supplying power from a remote power source to the repeaters.
A preferred example has the power couplers comprising a tail extending from the repeater, for splicing to the second underwater cable.
Any of the preferred features may be combined with any of the above mentioned aspects of the invention, as would be apparent to a skilled person. Other advantages may become apparent to a skilled person, particularly in comparison to other prior art of which the inventors are unaware.